Maimonides’ Mishnah Torah Sefer Madda Yesodei haTorah
(Book of Knowledge – Foundations of the Torah) – Chapter Nine
1) It is clear and explicit in the Torah that it is [God’s] commandment, remaining forever without change, addition, or diminishment, as [Deuteronomy 13:1] states: “All these matters which I command to you, you shall be careful to perform. You may not add to it or diminish from it,” and [Deuteronomy 29:28] states: “What is revealed is for us and our children forever, to carry out all the words of this Torah.” This teaches that we are commanded to fulfill all the Torah’s directives forever.
It is also said: “It is an everlasting statute for all your generations,” and [Deuteronomy 30:12] states: “It is not in the heavens.” This teaches that a prophet can no longer add a new precept [to the Torah].
Therefore, if a person will arise, whether Jew or gentile, and perform a sign or wonder and say that God sent him to:
a) add a mitzvah,
b) withdraw a mitzvah
c) explain a mitzvah in a manner which differs from the tradition received from Moses, or
d) if he says that the mitzvot commanded to the Jews are not forever, but rather were given for a limited time,
he is a false prophet. He comes to deny the prophecy of Moses and should be executed by strangulation, because he dared to make statements in God’s name which God never made.
God, blessed be His name, commanded Moses that this commandment is for us and our children forever, and, God is not man that He speak falsely.